One form of process and apparatus for producing hollow bodies from oriented thermoplastic material involves a mode of operation in which a blank which is produced for example by molding and which has a substantially cylindrical wall portion and a bottom portion is subjected to a treatment consisting of a plurality of steps, in which at least a part of the blank is subjected to a variation in temperature, deformed or shaped and then subjected to thermofixing.
Such a process is employed, particularly when using preforms of linear polyesters, for example polyethylene teraphthalate (PETP) or polybutylene teraphthalate (PEBTP), for example when there is a need to produce containers for materials to be contained therein, under a high pressure, such as for example carbon dioxide-bearing drinks.
That procedure may involve for example stretching the wall of a blank at a temperature below the vitreous transition temperature of the material forming same to provide an intermediate product or parison which is subjected to expansion at least over the major part of its longitudinal extent, and then subjected to a thermofixing operation.
In another alternative form of the procedure involved, an intermediate product is firstly produced by stretch blow molding, the plastic material which is shaped in that operation already being subjected to orientation in the axial direction and in the peripheral direction during the blow molding process. In that case it will essentially only then be necessary to provide for thermofixing of the wall portion of the intermediate product. Irrespective of the way in which the intermediate product is produced, the thermofixing operation is intended to ensure that the final product can be heated up to a given maximum temperature which is necessary in regard to containers for example for sterilising the content thereof, without the final product experiencing a significant alteration in its shape, due to the increased temperature involved in the heating operation. It is also desirable to ensure that the plastic material which forms the container or another kind of end product does not undergo shrinkage, under the effect of the heat applied.
Depending once again on the method of manufacture of the intermediate product, the thermofixed hollow body may also be subjected if necessary to further treatment, for example for the purposes of shaping the wall regions thereof which adjoin its opening in order in that way to produce a can which can then be closed by means of a conventional cover.